In recent years, aiming to solve a problem of the depletion of fossil resources, there have been actively developed functional materials that use biomasses, which are environmentally conscious materials that are continually available. Among them, cellulose, which is a main component of wood, is the most abundantly accumulated natural polymer material on Earth. Hence, cellulose has been expected as a key material for transition to a resource-recycling society. In wood, a bunch of tens or more of cellulose molecules form a highly crystalline fine fiber (microfibril) having a fiber diameter of nanometer order. In addition, a number of such fine fibers are hydrogen-bonded to each other to form a cellulose fiber to serve as a plant support. Thus, since natural cellulose contained in wood has a stable structure, the natural cellulose is insoluble in solvents but for a special solvent and is poor in formability. Hence, the natural cellulose contained in wood is difficult to handle as a functional material. Thus, approaches have been actively taken to utilize the cellulose fiber in wood by finely disintegrating it until at least one side of the structure thereof becomes nanometer-order length.
For example, as disclosed in Patent Literature 1, repeating machine processing using a blender or a grinder for wood cellulose, a finely-disintegrated cellulose fiber, that is, a cellulose nanofiber (hereinafter, referred to as CNF) can be obtained. It has been reported that the CNF obtained by this method has a minor axis diameter of 10 to 50 nm and a major axis diameter of 1 μm to 10 mm. This CNF has strength 5 or more times greater than that of steel, though the weight of the CNF is ⅕ of that of steel. The CNF has an enormous specific surface area of 250 m2/g or more. Hence, a number of examples have been already reported which use the CNF as a nanofiber for reinforcing resin (e.g. refer to Patent Literatures 2 and 3).
However, because there are essential problems that compatibility between hydrophobic general-purpose resin and a hydrophilic CNF is low and the formation of a complete complex is difficult, the CNF is not yet in practical use.
In addition, as finely-disintegrating technology using chemical treatment, one using acid hydrolysis is known. In addition, in recent years, a new method has been reported in which the relatively stable 2, 2, 6, 6-tetramethylpiperidinyl-1-oxy radical (TEMPO) is used as a catalyst to selectively oxidize the surface of a fine fiber of cellulose (e.g. refer to Patent Literature 4). An oxidation reaction using TEMPO as a catalyst (TEMPO oxidation reaction) enables an environmentally conscious chemical modification that progresses in a water system, at normal temperature, and under normal pressure. When the TEMPO oxidation reaction is applied to cellulose in wood, the reaction does not progress inside the crystal, and only the alcoholic primary carbon of a cellulose molecular chain of the surface of the crystal can be selectively converted into a carboxyl group.
Thus, cellulose single nanofibers (hereinafter, referred to as CSNF) can be obtained which are pieces of cellulose microfibril dispersed in a water solvent by electrostatic repulsion between carboxyl groups introduced to the surface of the crystal. A CSNF that can be obtained from wood by the TEMPO oxidation reaction, that is, a CSNF derived from wood, is a structure having a high aspect ratio where a minor axis diameter is about 3 nm, and a major axis diameter is several tens of nm to several tens of μm. Hence, it has been reported that a water dispersion liquid and a laminate of CSNF have high transparency. In addition, in an application example of using CSNF reported, CSNF is laminated on a transparent substrate to form a gas barrier film for use as a new plant-derived transparent packaging material (e.g. refer to Patent Literature 5).
However, there is a problem that since introducing carboxyl groups to CSNF greatly increases hydrophilicity of CSNF, the dense laminated structure of the CSNF inside the gas barrier film cannot be maintained under a humid temperate climate such as in Japan, thereby lowering gas barrier properties. Hence, practical use of the CSNF as a transparent packaging material is not yet promised at the present time.
As described above, various studies have been made for developing high-performance members using finely-disintegrated cellulose such as CNF and CSNF, which are carbon neutral materials. However, there are still a lot of problems for practical use.
When the size of a metal or a metal oxide decreases to a nanometer order, the metal or the metal oxide may exhibit physical and chemical properties different from those of a bulk state. This phenomenon is known as a so-called quantum size effect which includes lowering of the melting point and surface localized plasmon resonance in the case of metal nanoparticles.
Various applications of metal nanoparticles are expected to develop using the quantum size effect. To maintain the quantum size effect, metal nanoparticles are typically provided in a state of a dispersion liquid. However, since the specific surface area of the metal nanoparticle increases, the metal nanoparticles are easily flocculated with one another in the dispersion liquid, raising a problem of dispersion stability. If secondary particles are formed by the flocculation, the quantum size effect is lost. Hence, various additives are required to be used to prevent the metal nanoparticles from being flocculated with one another.
Meanwhile, in recent years, the use of metal nanoparticles having an anisotropic shape is receiving attention. For example, since the metal nanoparticles each having a plate shape or a rod shape have optical characteristics, electronic characteristics, magnetic characteristics, chemical characteristics, and mechanical characteristics different from those of spherical nanoparticles, anisotropic shape metal nanoparticles are expected to be applied to various fields. Use of metal nanoparticles having such an anisotropic shape is receiving attention in recent years as an approach to fully utilize the quantum size effect.
Among the metal nanoparticles having such an anisotropic shape, application of silver nanoparticles is especially expected. For example, it is known that spherical silver nanoparticles having a particle size of several nm to several tens of nm absorb light with a wavelength of approximately 400 nm due to localized surface plasmon resonance, and hence exhibit yellowish hue. However, anisotropically grown silver nanoparticles do not necessarily exhibit yellowish hue. For example, it is known that an absorption peak of plate-like silver nanoparticles is red-shifted. In this case, it has been confirmed that as an aspect ratio (i.e. particle size/thickness of particles) of the plate-like silver nanoparticles becomes larger, the absorption peak shifts to the long-wavelength side. That is, plate-like silver nanoparticles can be used as an optical material selectively absorbing light with a given wavelength. In addition, if an absorption wavelength is controlled to be within the visible light region, plate-like silver nanoparticles can be obtained which is brightly colored other than yellow, for example, red or blue. Thus, the plate-like silver nanoparticle is expected to be used as a functional color material. Furthermore, the absorption peak can be shifted to the near-infrared region outside the visible light region depending on the aspect ratio of the plate-like silver nanoparticles. In such a case, plate-like silver nanoparticles can also be applied to a near-infrared-absorbing material.
Note that, herein, near infrared rays indicate electromagnetic waves having a wavelength region (approximately between 700 nm to 2500 nm) close to that of the visible light in infrared rays. The near infrared rays have properties close to those of visible light. It is especially known that light having a wavelength in a region approximately between 700 nm to 1200 nm as included in sunlight, is easily absorbed into the surface of an object and then easily converted to thermal energy.
Such a near-infrared-absorbing material that can shield heat rays is a high-value-added functional material. For example, provision of a layer including the near-infrared-absorbing material to windows of a building or an automobile can obtain a heat shielding effect. That is, such a near-infrared-absorbing material which is expected to exert a power-saving effect due to the increase of cooling efficiency can contribute to resolving the power shortage problem in summer.
As described above, various synthetic methods have been proposed for plate-like silver nanoparticles having interesting properties as an optical material. Such synthetic methods include methods used for general purposes which include a synthetic method called a polyol process. According to this polyol process, ethylene glycol, which is one type of polyol, is heated together with a metal salt to 140 to 160° C. with a polymer capping agent to synthesize metal fine particles by means of reducing power of the generated glycol aldehyde. In this case, it has been reported that selecting reducing conditions and an appropriate polymer capping agent can induce anisotropic growth of the silver nanoparticles, thereby obtaining silver nanoparticles having various shapes. For example, Patent Literature 6 discloses an example of producing plate-like silver nanoparticles by using the polyol process.
In addition, Patent Literature 7 discloses a complex (metal nanoparticle supporting CSNF) in which metal nanoparticles are supported by CSNF, as a complex of metal fine particles and CSNF, which is one type of finely-disintegrated cellulose. Patent Literature 7 discloses an example of using the metal nanoparticles supporting CSNF as a catalyst.